The Moving Earth, Micro to Mega

When Caltech’s Nadia Lapusta creates computer models of earthquakes, she must integrate an astonishing range of data—on scales from thousands of kilometers down to microns and from millennia down to thousandths of a second. That’s because to understand the big and slow, she needs to understand the tiny and fast. “Large-scale earthquake ruptures—even those around 8 on the Richter scale—are ultimately happening in very narrow layers of granulated rock,” she says. In fact, where one side of a fault moves against the other, those layers are powdered so thin that a stack of a thousand grains would equal the thickness of a credit card. And although a fault can go eons between destructive quakes, the first slip that kicks off the shaking can take place in a blink.

Slow fault motions and earthquakes so small that they can only be detected by sensitive instruments such as seismometers occur all the time. Accounting for them is paramount for learning about the properties of a fault and getting a handle on the next big one, which likely will start with such a smaller-scale event.

“You have to understand the mechanics across the entire earthquake system, starting at the micrometer scale,” says Professor Lapusta, who holds a joint appointment in mechanical engineering and geophysics. “This is the challenge.”

She stands at a crossroads of disciplines, where knowledge from materials science about the behavior of rock under stress informs computational models from solid mechanics and applied math that are used to explore geophysical events. Her models describe the incredibly complex dynamics that occur where tectonic plates meet.

Taking full advantage of Caltech’s interdisciplinary culture and ready access to colleagues whose studies feed into her own science, she and her teammates have revealed fundamental truths about the movement of the earth that upend some traditional assumptions. This work has the potential to reduce the damage wrought by quakes and save lives.

Beyond What Can Be Seen

Lapusta aims to predict not what will happen when (which may be impossible), but rather what could happen under the specific conditions at the faults.

Her numerical models rely upon field observations, seismic monitoring, lab experiments, and theoretical science, while complementing those endeavors with a new perspective. The predictions expand researchers’ view beyond the limits of direct observation—which is important for events that occur across thousands of years.

“The problem is that large, destructive earthquakes are very rare, especially in any given area. The idea is to understand as much as we can about what could happen, using observations of past earthquakes and current smaller-scale phenomena as well as laboratory and theoretical knowledge.”

- Nadia Lapusta

As the Plates Move, Energy Builds

One of Lapusta’s signature findings started with the mystery behind a tragedy—the 2011 magnitude 9.0 Tohoku earthquake and tsunami in Japan. These events brought devastation to a supposed seismic safe zone.

An earthquake is essentially a release of forces and energy stored in the earth. Tectonic plates want to move against one another. When segments lock up, immense forces build and build until the plates finally slip and set the stored energy free, unleashing a quake.

In contrast, some sections of faults creep along steadily, at a rate of up to an inch a year. The traditional thinking was that such sections don’t store energy and can’t host quakes.

Except in Japan, it turned out that those sections were not so safe after all.

“The shallow portion of the fault was thought to be creeping, so researchers assumed that portion couldn’t have an earthquake,” Lapusta says. “It not only ruptured, but ruptured dramatically, with fault slips unmatched in the recorded history of earthquakes.”

In a 2013 paper, she provided an explanation, rooted in advanced laws of friction, showing that creeping faults do store energy and can unleash it under the right conditions. As Lapusta continues to explore, she already has created new knowledge that resonates right in Caltech’s backyard.

In the Middle

Quake-prone Los Angeles and San Francisco are separated by a creeping segment of the San Andreas Fault. Experts assumed that if a major seismic event were to strike one metropolis, the other—buffered by the creeping segment—would be unscathed, and thus able to step in to help with relief efforts. But Lapusta’s work suggests that a rupture from the north or south could transmit through the creeping middle. The findings have inspired additional field studies to estimate the likelihood of such an extreme event.

“If you have a large earthquake that spans both cities, one area will not be able to help the other,” she says. “And that scenario requires a different level of thinking.”

One of the eventual goals of Lapusta’s modeling is to develop the set of most likely scenarios for large, destructive earthquakes in California. In this way, her work—which is integral to Caltech’s broader focus on resilient megacities—would help infrastructure and emergency-response planners make choices that lessen damage from such unstoppable events.

A Powerful Partnership

Lapusta’s potentially game-changing work has been partially funded by Caltech’s Terrestrial Hazard Observation and Reporting (THOR) Center, established with a gift from Foster and Coco Stanback.

She credits the Stanbacks’ generosity—together with Caltech’s fearlessness in trying new ideas if the potential payoff is huge—with giving her the freedom to forge ahead in the relatively untrodden realm where one may devise predictive computational models of seismic activity.

“Working in a newer interdisciplinary area requires that kind of forward-looking philanthropic support,” Lapusta says. “With THOR, the Stanbacks said, ‘We will be pioneers,’ and provided funds that we can use to dream—and to attract the best graduate students and postdoctoral researchers.”

Just as big things start small with earthquakes, this partnership between philanthropists and Caltech’s intrepid investigators could help protect societies around the world from damaging earthquakes, one finding at a time.